1. Field of the Invention
The present invention relates to a power generation system using a fuel cell.
2. Description of the Prior Art
A fuel cell is an electrochemical device for converting fuel energy directly to electric energy by an electrochemical reaction. The fuel cell, depending on a charge carrier used, is broadly divided into a phosphoric fuel cell, a melt carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte fuel cell (hereinafter, abbreviated to PEFC), and an alkaline fuel cell.
Among these various fuel cells, the PEFC enables high current density power generation and operation at a comparatively low temperature, so that applications to various uses including a power source for a mobile body are anticipated.
For an electrolyte of the PEFC, an ion exchange membrane with a thickness of several tens μm to hundred and several tens μm is used. The ion exchange membrane has a general structure that a side chain having the sulfonic group is bonded to fluorocarbon composing a main chain. The ion exchange membrane has proton conductivity, thus the membrane material is required to contain moisture. The reason is that the sulfonic group takes a cluster structure in the material, and the clusters are connected by a channel, and by conduction of protons (H3O+) in the channel, the material shows proton conductivity, though to follow this mechanism, existence of water is necessary.
Therefore, when operating the PEFC, a system that gas to be supplied contains moisture for moistening the electrolyte is general. When the PEFC generates power, it generates water by the chemical reaction thereof. If collected generated water can be used as moistening water, a system requiring no feed water from the outside can be formed and the constituent devices can be simplified.
When the constituent devices can be simplified, not only the manufacturing cost can be reduced but also the system volume can be made smaller, and particularly when the PEFC is used as a power source for a mobile body, the advantage is increased.
To improve the operability of the fuel cell system, it is preferable to realize a rapid start characteristic.
To start the system at high speed, it is necessary to promptly increase the cell temperature to a preset temperature. When separately installing a heater mechanism for increasing the temperature of the cell, it is extremely inefficient in respect of efficiency and volume, so that it is generally difficult to load it in a power source for a mobile body. Therefore, in a system having no special cell temperature rise mechanism, as a heat source for increasing the temperature of the cell, heat generated by power generation is used. To form the fuel cell, several hundreds separators are generally used as one of the main materials. When using carbon separators comparatively thick such as several mm or more, the fuel cell is long in the lamination direction. Simultaneously, in correspondence to an increase in the stack volume, the heat capacity also increases. Namely, the temperature rise speed at time of power generation is low and rapid start is difficult.
Improvement of the start characteristic at low temperature is described in Japanese Patent Laid-open No. 2005-190744.